1. Field of the Invention
The present invention relates to undersea pipes, and more particularly to pipes suitable for being installed empty at great depth, in particular deeper than 1000 meters (m).
2. Description Of Related Art
The technical field of the invention is that of manufacturing and installing pipes for undersea operation of fields of oil, gas, or other material that is soluble or fusible or of a suspension of mineral material.
The present invention relates more particularly to pipes for use in ultra-great depths, i.e. 1000 m to 2000 m, or even 3000 m or more.
The invention also relates to the field of transporting gas by means of undersea pipes.
It relates more particularly to developing production fields installed at sea, off-shore. The main application of the invention is thus in the field of producing petroleum.
The present invention relates to the known field of connections of the undersea pipe type installed either on the sea bed or else rising towards the surface to an anchored floating support.
The production fields considered below in the present description are oil fields, and once the depth of an oil field becomes large, it is general practice to operate it from floating supports. Wellheads are often distributed over the entire field, and production pipes together with water injection lines and monitoring and control cables are placed on the sea bed leading to a fixed location, with the floating support being positioned on the surface vertically thereabove.
Pipes are generally laid from the surface using specialized barges and the so-called “S” technique providing the depth of water is not too great, e.g. 300 m to 500 m, or the so-called “J” technique when the water is deeper. The terms “S” and “J” come from the appearance of the curve taken up by the pipe between the laying barge and the bottom, which curve is respectively S-shaped or J-shaped.
While it is being installed, the pipe is laid empty so as to take advantage of the additional support provided by buoyancy thrust, thereby considerably reducing the tension that is required at the barge in order to hold the pipe that is being assembled.
Thus, when the pipe rests on the bottom, because it is empty and in direct communication with the surface, its inside pressure corresponds substantially to atmospheric pressure, whereas its outside pressure corresponds to the pressure at the bottom of the sea, i.e. a pressure of approximately 10 bars per 100 m of depth.
Thus, the pipe must be capable of withstanding a bottom pressure of about 150 bars at a depth of 1500 m and of about 350 bars at a depth of 3500 m. This pressure tends to cause the pipe to implode, with such implosion taking place suddenly once the limit pressure has been exceeded.
It would be possible to lay the pipe while full in order to avoid the above drawback, however the tension needed at the head of the pipe would then rapidly become excessive, and in the case of oil production, because gas can come from an oil well, such arrivals of gas would run the risk of completely filling considerable lengths of pipe, which would return to the same problem as above whenever the pipe is depressurized at the surface.
For shallow or medium depths, pipes are thus generally dimensioned to withstand an internal pressure in service and their implosion behavior is verified so as to avoid future incidents. However, at great depths and ultra-great depths, the non-implosion criterion becomes predominant and pipes have to be dimensioned relative to this criterion.
While laying pipes in an S-shape or a J-shape, whenever the position of the vessel is unstable, the resulting deformation in the S- or J-shaped curve runs the risk of leading to an unacceptable increase in the curvature of the pipe, and that can degenerate into the pipe buckling locally or “kinking”. Buckling degenerates almost instantly into implosion of the pipe. The implosion propagates very quickly to all of that portion of the pipe which is in suspension, and also to all of the pipe which is laid on the sea bottom. This phenomenon can be avoided by increasing the wall thickness of the pipe or by using higher-performance steel, but increasing the quantity of material used has the effect not only of increasing the cost of the pipe, but also of increasing the tension needed for laying it.
Because it is desirable to optimize overall cost, it is preferred to install localized reinforcement referred to as “buckle” arrestors at regular intervals, i.e. to install anti-implosion rings having the function of preventing an implosion initiated at a localized buckle from propagating. These anti-implosion rings are generally localized portions of the pipe having extra wall thickness, these portions occupying a length of 30 centimeters (cm) to 1 m, and being distributed every 200 m along the entire pipe, for example. Thus, in the event of an incident occurring during laying, the implosion is restricted to the portion that extends between two rings. The pipe can then be raised until the damaged zone is reached, the damaged zone removed, and laying continued.
In certain fields, pipes need to be insulated so that the petroleum effluent reaches the surface at a temperature higher than some minimum temperature in order to avoid the viscosity of the crude oil increasing and also in order to avoid paraffins or hydrates forming. It is thus desirable to avoid the temperature of the crude oil dropping to below 30° C.-40° C. prior to reaching the surface. Because sea water at great depth is at a temperature of about 4° C., numerous very high performance insulation systems have been developed in order to achieve this objective, and above all in order to maintain the temperature of the crude in the event of production being interrupted in untimely manner. It is very difficult to restart an installation in the event of a localized obstruction, and restarting is even more difficult if the obstruction has become generalized.
Amongst the techniques used for performing the insulation function, some are known as pipe-in-pipe or “PiP”. This involves an inner pipe conveying the hot fluid being installed inside an outer protection pipe, with the space between the two pipes being either merely evacuated or else filled with lagging, optionally confined in a vacuum. PCT/FR00/03200 describes that type of assembly comprising a coaxial pipe with an outer pipe containing an inner pipe, with the pipes being interconnected by centralizing mechanical links, and with the space between said inner and outer pipes preferably containing an insulating material.
In that type of pipe, the fact of the space between the pipes being either evacuated or else under substantially atmospheric pressure requires the inner pipe to be dimensioned mainly in order to withstanding bursting at service pressure, while the outer pipe is dimensioned mainly to be able to withstand implosion at bottom pressure.
In PiPs, the inner pipe is at the same temperature as the fluid, i.e. at high temperature, whereas the outer pipe is at sea bottom temperature, i.e. about 4° C. This gives rise to differential expansion between the inner pipe and the outer pipe, and that can generate considerable forces, possibly as great as several tens or even several hundreds of (metric) tonnes (t) which then act on the ends and which must be contained by rugged connection structures capable of preventing one of the pipes moving axially in unwanted manner relative to the other. These phenomena of bottom pressure and differential expansion are known to the person skilled in the art in the field of oil production, and they are not developed in greater detail herein.
U.S. Pat. No. 4,261,671 proposes making multiple corrugations that are regular and close together, in a circular or a spiral configuration in order to increase the ability of a pipe to withstand pressure. Those multiple corrugations comprise symmetrical or complementary shapes and backing shapes following one another in continuous manner. Such corrugated or spiral shaping is generally performed by hydraulically expanding a tubular wall from the inside, and it is very difficult to perform, particularly on pipes of considerable wall thickness.
Given that the corrugations follow one another continuously and are therefore very close together, it is necessary to insert a coating inside the pipe in order to smooth the inside surface of its wall, thus serving to fill in the too numerous recesses that otherwise run the risk of disturbing fluid flow within the pipe.
French patent No. 2 781 034 describes a lightweight pipe that is insulated and mechanically reinforced, using a corrugated outer pipe resting on inner tubing, with lagging then filling the voids. Such a pipe then presents considerable thermal bridges because of the contact between the inner and outer pipes, and furthermore, since it is made out of light-weight materials, it is incapable of performing the functions expected of a pipe that is for use in ultra-great depths at sea.
French patent No. 2 808 864 describes a pipe having improved resistance to buckling in which successive necks are made that are spaced apart from one another at a distance of 0.25 to 3 times the diameter of the tubular wall, and that are thus in smaller numbers than the corrugations in the embodiments of U.S. Pat. No. 4,261,671. Nevertheless, the necks are obtained by hollow forming of the stamping type from the outside, and the inside diameter of the pipe is correspondingly narrowed. In that type of pipe, since the diameter of the tubular portion of the pipe is greater than the flow passage through the necks, in order to withstand a given pressure it is necessary for wall thickness to be greater than for the version described in the preceding patent, in order to obtain the same inside flow diameter available to the fluid inside the pipe.
In both U.S. Pat. No. 4,261,671 and FR 2 808 864, making corrugations or necks in regular manner all along the pipe is very difficult if it is also desired to conserve a pipe that is rectilinear and of inside diameter that is substantially constant.